A Contour Shape Description Method Via Transformation to Rotation and Scale Invariant Coordinates System

Abstract

Rotation and scale variations complicate the matters of shape description and recognition because these variations change the location of points composing the shape. However, some geometric invariant points and the relations among them are not changed by these variations. Therefore, if points in image space depicted with the x-y coordinates system can be transformed into a new coordinates system that are invariant to rotation and scale, the problem of shape description and recognition becomes easier. This paper presents a shape description method via transformation from the image space into the invariant feature space having d- and c-axes: representing relative distance from a centroid and contour segment curvature (CSC) respectively. The relative distance describes how far a point departs from the centroid, and the CSC represents the degree of fluctuation in a contour segment. After transformation, mesh features were used to describe the shape mapped onto the d-c plane. Traditional mesh features extracted from the x-y plane are sensitive to rotation, whereas the mesh features from the d-c plane are robust to it. Experimental results show that the proposed method is robust to rotation and scale variations.